High voltage tantalum anode and method of manufacture

ABSTRACT

Tantalum powders produced using a tantalum fiber precursor are described. The tantalum fiber precursor is chopped or cut into short lengths having a uniform fiber thickness and favorable aspect ratio. The chopped fibers are formed into a primary powder having a controlled size and shape, narrow/tight particle size distribution, and low impurity level. The primary powder is then agglomerated into an agglomerated powder displaying suitable flowability and pressability such that pellets with good structural integrity and uniform pellet porosity are manufacturable. The pellet is sintered and anodized to a desired formation voltage. The thusly created capacitor anode has a dual morphology or dual porosity provided by a primary porosity of the individual tantalum fibers making up the primary powder and a larger secondary porosity formed between the primary powders agglomerated into the agglomerated powder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/479,689, filed on Sep. 8, 2014, now U.S. Pat. No. 9,312,075,which claims priority to U.S. provisional patent application Ser. No.61/874,573, filed on Sep. 6, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a valve metal anode for acapacitor, and more particularly, to an electrolytic capacitorcomprising an anode formed from a pressed pellet of tantalum fibers. Thetantalum fiber pellet is sintered and then anodized into a high voltageanode at formation voltages up to 550V.

2. Prior Art

Development of powders suitable for making a tantalum capacitor has beena focus of both capacitor producers and tantalum processors.Historically, the intent has been to delineate requirements for tantalumpowder that will result in capacitors having reliable performance,particularly in demanding high voltage applications such as cardiacdefibrillation. It is understood that demanding applications, such ascardiac defibrillation, require tantalum powders having suitable surfacearea, high purity, uniform feature size, optimized shrinkage, favorableflowability and pressability, and green pellet strength.

Wet tantalum capacitors have been used in implantable cardiacdefibrillators as the output energy storage capacitor for delivering thetherapeutic electrical shock to the heart to stop a defibrillationevent. These shocks are generally delivered at voltages ranging fromapproximately 650 volts to 950 volts. To achieve therapy delivery atsuch high voltage levels, between three and four tantalum capacitors aretypically used in the output stage of the defibrillator.

Several advantages are associated with reducing the number ofcapacitors. For example, fewer capacitors required for energy storagesimplifies the device charge and discharge circuits. Also, a reductionin the number of capacitors results in a reduction in the number ofcomponents in the device. Fewer components mean that the potential forperformance issues decreases, thereby favorably impacting reliability.Other advantages of fewer components are more efficient assembly andlower cost.

Accordingly, one purpose of this invention is to develop a manufacturingprocess for tantalum anodes that are suitable for building anelectrolytic capacitor for incorporation into a cardiac defibrillator.The manufacturing processes include pressing, sintering and formingsteps. It is also the purpose of this invention to fabricate a tantalumanode that is capable of being formed at higher voltages than iscurrently known in the prior art. An anode for high voltage applicationssuch as described within must also have a pore structure and internalsurface area that allows for low ESR and high capacitance.

It is known in the art that ESR is related to energy loss. It is alsoknown that for a capacitor, energy loss during charging and dischargingimpacts capacitor efficiency. Hence, a lower ESR of an anode made inaccordance with the present invention improves the efficiency of thecapacitor. This is of significance in cardiac defibrillation asdischarge of the capacitor delivers the energy needed to return theheart to normal rhythm. The improved efficiency achieved by the presentinvention enables delivery of energy and higher voltages, and allows forsmaller batteries to be used in implantable defibrillators due to lessenergy being required to charge the capacitors. Improvement in thecapacitance per unit volume of an anode of the present invention allowsmore charge to be stored per unit volume, resulting in a capacitor thatstores more energy per unit volume.

When tantalum powders are formed into a porous anode body and thensintered for use in an electrolytic capacitor, it is known that theresultant anode capacitance is proportional to the specific surface areaof the sintered porous body. The greater the specific surface area aftersintering, the greater the anode capacitance (μFV/g) is. Since the anodecapacitance (μFV/g) of a tantalum pellet is a function of the specificsurface area of the sintered powder, one way to achieve a greater netsurface area is by increasing the quantity (grams) of powder per pellet.However, with this approach cost and size increase considerably.Consequently, cost and size considerations dictate that tantalum powderdevelopment focus on means to increase the specific surface area of thepowder itself.

Prior art methods for increasing the specific surface area of tantalumpowder include flattening the powder particles into a flake shape orspherical granulation to produce ovular particle shapes. For example,U.S. Pat. No. 4,940,490 to Fife et al., U.S. Pat. No. 5,211,741 to Fifeand U.S. Pat. No. 5,580,367 to Fife disclose flaked tantalum powders andmethods for making the flaked powders. FIG. 1 is an SEM photograph at5,000× showing flake tantalum particles according to the prior art.

However, efforts to further increase specific surface area by makingthinner tantalum flakes have been hindered by concomitant loss ofprocessing characteristics. For example, several of the majordeficiencies of very thin tantalum flake are poor flow characteristics,poor pressability and low green strength, and low forming voltages.Moreover, increasing particle size using spherical granulation stilltends to result in particles that are finer than desirable. Theresultant pore size and structure of pellets made from sphericalparticles tend to be smaller. Pellet structure damage during hightemperature formation is a further area of concern.

One commonly used tantalum powder having relatively large particles iscommercially available from H. C. Starck under the designation QR-3.This so called EB melt-type tantalum powder permits anodes to be madewith relatively larger pore structures. The larger pore structures allowformation electrolytes to cool the interior of the pellets duringformation. However, the relatively small surface area of these largeparticle size powders does not result in anodes of high capacitance perunit volume. That is because the relatively large particle size resultsin excessive amounts of tantalum metal remaining after formation oftantalum oxide. FIG. 2 is an SEM photograph at 1,000× showing EB melttantalum particles according to the prior art.

Another commonly used material is available from H. C. Starck as sodiumreduced tantalum powder under the designation NH-175. Because of itsrelatively higher surface area, this material is known to produce anodeswith higher capacitance than QR-3 powders. However, because of itssmaller feature size and broad particle size distribution, NH-175powders are also known to produce anodes with smaller pore structures.The smaller pore structure makes internal cooling of anode pelletsduring anodization more difficult, and limits the formation voltagesthat these anodes can achieve. If formation voltage gets too high, manyof the NH-175 tantalum particles are formed completely through, leavingno conductive pathways behind the tantalum oxide. FIG. 3 is an SEMphotograph at 5,000× showing a sodium reduced NH-175 tantalum powderagglomerate according to the prior art.

Purity of the powder is another important consideration. Metallic andnon-metallic contamination tends to degrade the dielectric oxide film intantalum capacitors. While high sintering temperatures serve to removesome volatile contaminants, not all may be removed sufficiently,resulting in sites having high DC leakage. High DC leakage is known tocontribute to premature electrical failures, particularly in highvoltage applications. Further, high sintering temperatures tend toshrink the porous anode body, thereby reducing its net specific surfacearea and thus the capacitance of the resulting capacitor. Therefore,minimizing loss of specific surface area under sintering conditions,i.e., shrinkage, is necessary in order to produce high μFV/g tantalumcapacitors.

Flowability of tantalum powder and green strength (mechanical strengthof pressed, unsintered powder pellets) are also importantcharacteristics for a capacitor producer. Not only does flowabilityprovide for efficient pellet production, it provides for high volume,automated pellet production. Flowability of agglomerated tantalum powderis even more essential to production efficiency and proper operation ofautomatic pellet presses. Sufficient green strength permits handling andtransport of a pressed product, e.g., pellet, without excessive breakageor pellet damage (detectable and undetectable) that could affectproduction reject rates and finished product performance.

Accordingly, what is needed is a tantalum fiber of a strictly controlleddiameter such that sufficient metal remains after formation to provide aconductive matrix behind the dielectric oxide. Because of the tightlycontrolled fiber diameter according to the present invention, fiberdiameter can be minimized to a greater extent than with other prior artpowder types. By minimizing fiber diameter while ensuring that tantalumis not totally consumed during formation, the dielectric surface areacan be maximized without isolating dielectric area due to loss oftantalum substrate.

In that respect, a tantalum anode according to the present invention isdistinguishable from the prior art. Regardless whether the tantalum isof a flake or spherical shape manufactured by the beam melt (QR-3powder) or sodium reduction processes (NH-175 powder), the presentinvention uniquely discloses the pressing and sintering of anagglomerate of tantalum fibers having a tightly controlled aspect ratio.The result is an electrode pellet having a dual morphology and that iscapable of being anodized into a capacitor anode at formation voltagesup to 550V.

SUMMARY OF THE INVENTION

In order to generate high voltage anodes having high capacitance, andtherefore high energy density, anodes having high per unit surface areamust be fabricated. High surface area anodes must also have porestructures that allow for good internal cooling during anode formation,and have lower ESR both during formation and subsequently while in usein the finished capacitor. The use of tantalum anodes made from tantalumfibers according to the present invention improves on these issues.

In the present invention, the diameter of the tantalum fibers used togenerate the finished pellet is tightly controlled. First, the tantalumfibers are divided into desired lengths (up to 50 microns) to form arandomly oriented, porous powder (primary powder). The primary powder issubsequently subjected to an agglomeration process to thereby form anagglomerated powder of the tantalum fibers. An exemplary agglomerationprocess is described in U.S. Pat. No. 5,217,526 to Fife wherein tantalumfibers of the primary powder are heat treated at 1,000° C. for 30minutes. The random agglomerate structure is stabilized byfiber-to-fiber bonding (sintering). Another agglomeration process isuseful with the present invention is described in U.S. Pat. No.4,017,302 to Bates et al. The '526 and '302 patents to Fife and Bates etal. are incorporated herein by reference. The resulting agglomeratedpowder has very narrow particle and pore size distribution. Theagglomerated powder can be pressed into a pellet of a desired shapecomprising the tantalum fibers of the tightly controlled diameter usedto make the primary powder, but with a pellet structure provided withlarger sized pores provided by the agglomeration of the primary powder.This so called “dual morphology” or dual porosity pellet structureallows for better electrolyte penetration. Better electrolytepenetration aids in both cooling of the pellet during formation as wellas lowering the ESR of the pellet when used as an anode in a capacitor.

These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following detailed description and the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM photograph at 5,000× showing flake tantalum particlesaccording to the prior art.

FIG. 2 is an SEM photograph at 1,000× showing EB melt tantalum particlesaccording to the prior art.

FIG. 3 is an SEM photograph at 5,000× showing a sodium reduced tantalumpowder agglomerate according to the prior art.

FIG. 4A is a transverse cross section of a primary billet 10 used in theproduction of tantalum fibers.

FIG. 4B is a cutaway view of the primary billet 10 shown in FIG. 4Arevealing the longitudinal disposition of the billet components.

FIG. 5 is a schematic depiction of the transverse cross section of thesecondary billet 22 used to make tantalum fibers.

FIG. 6 is a schematic depiction showing a cylindrical body containing aplurality of tantalum fibers.

FIGS. 7 and 8 are SEM photographs at 70× and 500×, respectively, showingthe present tantalum powder as coarse agglomerate with high surface areaand small pore structure.

FIGS. 9 and 10 are SEM photographs at 1,000× and 2,000×, respectively,showing the present tantalum powder having a relatively uniform fiberdiameter.

FIG. 11 is an SEM photograph at 4,000× showing the present tantalumpowder having good random 3-D fiber orientation.

FIG. 12 is an SEM photograph at 10,000× showing the present tantalumpowder having good uniform inter-particle spacing.

FIG. 13 is an SEM photograph at 25× showing the pore structure of apellet pressed from a tantalum powder according to the presentinvention.

FIG. 14 is an SEM photograph at 200× showing the pore structure of apellet pressed from a tantalum powder according to the presentinvention.

FIG. 15 is a perspective view of a capacitor 100 according to thepresent invention.

FIG. 16 is a perspective view of a dual anode/cathode assembly for thecapacitor 100 shown in FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As defined herein, a fiber is a very fine thread or threadlike tantalumfilament of indefinite length. A primary powder is a mass of loosetantalum fibers. An agglomerated powder is a mass of primary powderswhich have been bonded together through an agglomeration process byheating the primary powder under chemically non-reactive conditions to atemperature sufficient to form stabilized fiber-to-fiber bonding. Theresulting tantalum bodies consist essentially of short relativelyuniform diameter tantalum fibers, bonded and randomly oriented in asubstantially non-aligned, porous array.

As shown in FIGS. 4A and 4B, one process for manufacturing tantalumfibers for fabricating a tantalum anode that are useful for buildingcapacitors according to the present invention begins as a primary billet10 comprising tantalum rods 12 that have been inserted into holes 14drilled longitudinally into a copper matrix 16. In the matrix, copperseparates the tantalum rods 12 from each other. The rods 12 runlongitudinally through the body of the billet and are substantiallyuniform in diameter and aligned in parallel. After assembly, a coppernose 18 and tail 20 are welded onto the primary billet 10, and thebillet is then evacuated and sealed. At this point the primary billet 10may optionally be hot or cold isostatically pressed in order to collapseany void space, thereby promoting filament uniformity.

The primary billet 10 containing the tantalum rods 12 in a copper matrix16 is extruded at elevated temperature at a diameter reduction ratio ofapproximately 6:1. The resulting rod is cropped and then drawn down torestack diameter. Annealing may optionally be performed during drawingshould the wire become too stiff or breakage occurs. Annealingtemperatures for tantalum are typically in the range of 900° C.

At restack diameter, the composite wire is cut into lengths for assemblyinto a secondary billet 22 (FIG. 5). The sub-elements 24 made from theprimary billet are stacked together with copper rods. The copper rodsare used to form a copper core 26 and an outer annulus 28. The core 26and the outer annulus 28 make leaching of the final composite lessdifficult. An outer tantalum sheet 30 covers the assembly ofsub-elements and copper rods. The sheet 30 is the same length as therods and it completely surrounds the filament array. Outside thecylinder of tantalum sheet is an outer copper can 32.

The secondary billet 22 is assembled, a nose and tail (not shown) arewelded into place, and the billet is evacuated and sealed. The sealedbillet is optionally prepared for extrusion by hot or cold isostaticpressing in order to collapse any void space within the billet and topromote filament uniformity. After isostatic pressing, the secondarybillet is machined to fit the extrusion liner. The billet is thenextruded at elevated temperature at a diameter reduction ratio of 6:1.

The extruded rod is cropped, and the rod is then drawn to a diameterwhere the tantalum filament diameter is 5 microns or less. Again,annealing steps may be employed if necessary. At final size, thecomposite tantalum wire is cut into short lengths as required.

The cut sections are immersed in a solution of nitric acid and water,and for a period of time sufficient for the acid to fully leach out thecopper core 26 and outer annulus 28, leaving copper tantalum filamentsand the tantalum sheath 32 behind. Since the tantalum filaments arecomparatively tightly spaced, the copper core 26 and annulus 28 etchaway much more rapidly than the copper separating the filaments. As aresult, the acid eventually surrounds the annulus of tantalum filaments,and then attacks the filament matrix from all directions, rather thanjust from the ends of the cut sections. The total leaching time dependprimarily upon the composite wire diameter and length, with smallerdiameters and greater lengths requiring longer times. After leaching, aplurality of fine tantalum filaments (<5 micron diameter) surrounded bya thin tantalum tube 36 is left behind. The tube 36 is removed, leavingthe tantalum filaments 34 behind. The tantalum filaments or fibers areof an optimum diameter range of 0.5 μm to 2.5 μm. That the tantalumfilaments are of a strictly controlled diameter range is important forfabrication of an anode according to the present invention. Not onlymust the tantalum filaments be of a prescribed diameter, the abovepreparation process provides filaments of a narrow length range and highpurity substantially free of copper.

For more detail regarding production of tantalum filaments that areuseful in the present invention, reference is made to U.S. Pat. Nos.4,674,009, 5,034,857, 7,480,978 and 8,858,738, all to Wong and U.S. Pat.No. 5,869,196 to Wong et al., which are incorporated herein byreference. Other exemplary processes for forming tantalum filaments orfibers useful in the present invention are disclosed in U.S. Pat. Nos.3,277,564, 3,379,000, 3,394,213, 3,540,114, 3,567,407, 3,698,863,3,742,369, 3,800,414, 4,502,884, 5,217,526, 5,284,531, 5,245,514, and5,306,462, the contents of which are incorporated by reference herein.

FIGS. 7 to 12 are SEM photographs at various magnifications showingtantalum fibers according to the present invention.

The thusly produced tantalum fibers allow for the generation of anodeshaving a dual morphology. This dual morphology provides a higher surfacearea material compared to prior art powders. The term “dual morphology”means there are two pore structures within the pressed tantalum anodepellet. First, the previously described tantalum fibers that have beendrawn in a tightly controlled manner to an optimum diameter range of 0.5μm to 2.5 μm are chopped to an optimum length ranging from 5 μm to 50μm. The chopped fibers have a length-to-width aspect ratio ranging from2 to 100. A more preferred aspect length-to-width ratio ranges from 10to 40. These fibers form a primary powder as a loosely packed mass ofthe tantalum fibers.

Then, an agglomerated powder is formed by subjecting the primary powderto an agglomeration process. During agglomeration, for example, thetantalum fibers of the primary powder are heat treated at 1,000° C. for30 minutes. This serves to stabilize the agglomerate structure throughfiber-to-fiber bonding (sintering). As previously discussed, exemplaryagglomerating processes are described U.S. Pat. No. 4,017,302 to Bateset al. and U.S. Pat. No. 5,217,526 to Fife.

Thus, agglomeration serves to bond the primary powders together intobodies consisting essentially of the short (5 μm to 50 μm) tantalumfibers, bonded and randomly oriented in a substantially non-aligned,porous array. The tight diameter distribution of the tantalum fibers inthe agglomerated powder provides a relatively high surface area that isoptimally suited to provide high capacitance per unit volume of apressed pellet. The relatively small pores in the primary powder,however, cause higher ESR than is desirable. The agglomerated powdercompensates for this by providing more open pore structure than in theprimary powder when used to manufacture an anode pellet. Thus, the dualmorphology or dual porosity is the result of the primary porositybetween the individual tantalum fibers making up the agglomerate powderand the larger secondary porosity formed between the agglomerated powderin the body of the anode.

Table 1 below provides more detail on powder particle characteristics.As used this table, a diameter is defined as a straight line passingfrom side to side of a tantalum fiber, through its center.

TABLE 1 Primary Powder (Fiber) Fiber Diameter 0.5-2.5 μm Fiber Length5-50 μm L/D Aspect Ratio  2-100 (Preferred Range) 10-40 AgglomeratedParticle Agglomerate Density 1.5-4.5 g/cc (Preferred Range) 2.5-3.5 g/ccAgglomerate Dia. Distribution d50: 200-500 μm d10: 74 μm d90: 1,000 μmAgglomerate Pore Size d50: 1-5 μm (Preferred Range) d50: 2-3 μmAgglomerate Pore Distribution d10: 0.5 μm d90: 20 μm

Next, the tantalum agglomerate of a pore size from about 1 μm to 5 μm,preferably about 2 μm to about 3 μm comprising a random distribution offibers of a diameter of from about 0.5 μm to about 2.5 μm and of alength of from about 5 μm to about 50 μm is pressed into a pellet of adesired shape. The pellet contains thousands of the tantalumagglomerates (a plurality). The pressed anode pellet has a relativelylarger pore structure with lower resistance pathways within. Moreover,this open, lower resistance structure allows for better cooling of theanode pellet during the anode formation process avoiding the issuesassociated with prior art tantalum anode formation.

A “pellet”, as the term is used herein, is a porous mass, body orstructure comprised of agglomerated tantalum powder having the size andshape characteristics set forth in Table 1. Green strength is a measureof a pellet's mechanical strength prior to sintering. The term“pressability” describes the ability of a tantalum powder to be pressedinto a pellet. Tantalum powder that can be formed into pellets thatretain their shape with sufficient green strength to withstand ordinaryprocessing and manufacturing conditions without significant breakagehave good pressability.

FIGS. 13 and 14 are photographs showing that the presentuniformly-shaped tantalum fibers of the primary and agglomerated powdersare suitable for forming pellet structures having relatively lowcompaction densities. The tantalum fibers deform under smaller forcesand interlock with adjacent fibers to provide pellets with improvedgreen strength. The high green strength also has an added benefit ofpermitting an agglomerate with little to no fines (−200 mesh) material.In commercially available tantalum powders, fine material oftenrepresents 50% or more of the overall powder volume. In those instances,the fine powder particulate fills the spaces between the pores and serveto densify the anode structure. Thus, the fibrous powder allows for anagglomerate particle having a narrow size distribution with few to nonelittle fines that when pressed under relatively low force and thensintered, provides an open network of pores throughout the anode.

Table 2 below provides more detail on pellet pressing parametersaccording to the present invention.

TABLE 2 Pellet Pressing Conditions Pressed Density 3-8 g/cc (PreferredRange) 4-6.5 g/cc

Following pellet pressing, the green tantalum structure is sintered byheating to form a coherent body. Sintering is a high temperature processby which two fibers touching each other at a point contact coalesce orare fused together. As known by those skilled in the art, neck growth atthe point contact grows to create a new grain boundary. With sufficienttime, the contacting surfaces will eventually coalesce into a singlelarge contact or contact neck. An exemplary sintering protocol isdescribed in U.S. Pat. No. 6,965,510 to Liu et al., which is assigned tothe assignee of the present invention and incorporated herein byreference. The '510 Liu et al. patent describes sintering a pressedvalve metal pellet at a relatively high temperature, but for arelatively short time.

Table 3 below provides more detail on pellet sintering parametersaccording to the present invention.

TABLE 3 Pellet Sintering Conditions Sinter Temperature 1,200-2,200° C.(Preferred Range) 1,500-1,850° C. Sinter Time 0.1-120 minutes (PreferredRange) 1-10 minutes Vacuum Level <1 × 10⁻⁴ Torr (Argon) (PreferredRange) <1 × 10⁻⁵ Torr (Argon) Sintered Pellet Sintered Density 4-9 g/cc(Preferred Range) 5-8 g/cc Pellet Pore Size Inter granule d50: 1-5 μm(Preferred Range) Intra granule d50: 30-80 μm Inter Granule Pore d10:0.5-2 μm Distribution d90: 3-10 μm Intra Granule Pore d10: 20-40 μmDistribution d90: 60-100 μm

After sintering, the tantalum body is anodized to a desired formationvoltage in an anodizing electrolyte. A suitable anodizing electrolyte isdescribed in U.S. Pat. No. 6,231,993 to Stephenson et al., which isassigned to the assignee of the present invention and incorporatedherein by reference. An exemplary anodizing electrolyte useful with theformation protocols described in the '993 patent consists of, by volume:about 55% ethylene glycol, about 44.9% to about 43.5% deionized waterand about 0.1% to about 1.5% H₃PO₄. Such an electrolyte has aconductivity of about 2,500 μS to about 2,600 μS at 40° C. Theconductivity of the formation electrolyte can be increased to therebyreduce heat generation inside the anode pellet by using an aqueouselectrolyte of H₃PO₄ having a conductivity up to about 20,000 μS at 40°C.

It is believed in the industry that locally excessive temperatures andinsufficient material transport in porous valve metal bodies duringanodizing (especially for anodization of high voltage, relatively large,pressed and sintered tantalum powder pellets) causes breakdown or pooranode electrical properties. Because a pressed tantalum pellet isporous, anodizing electrolyte is able to flow into the pellet where itbecomes heated during the anodization process. Heated electrolyte thatis unable to readily flow out of the pellet can cause the temperature ofthe electrolyte within the porous structure to increase. It is believedthat heated electrolyte in the porous structure is responsible forcracks, fissures and similar imperfections as well as crystalline oxideformed in the oxide coating and inside the tantalum pellet. In additionto contributing to high DC leakage, these faults degrade the voltage towhich the anode can be charged before breakdown occurs.

A preferred anodizing method that helps prevent the accumulation ofheated electrolyte inside the tantalum body is to taught in U.S. Pat.No. 6,231,993 to Stephenson et al., which is assigned to the assignee ofthe present invention and incorporated herein by reference.

U.S. Pat. No. 7,727,372 to Liu et al. describes subjecting the tantalumbody to a current that decreases over time, a formation voltage thatincreases over time to a level below the voltage from the power supplyand a power level that is self-adjusted to a level that decreasesexcessive heating in the structure. This patent is assigned to theassignee of the present invention and incorporated herein by reference.A preferred formation voltage range is at least 200 V up to 550V. A morepreferred range is from 235 V to 480 V. A most preferred formationvoltage range is at least 300 V up to 550 V.

Thus, a unimodal agglomerated powder with limited particle sizedistribution according to the present invention has multiple advantagesin the formation of a high voltage tantalum anode. A limited particledistribution provides interstitial gaps between the particles thatpromotes electrolyte flow and cooling of the anode during formationsteps. Maintaining a low internal anode temperature during formation iscritical for inhibiting growth of crystalline tantalum oxide.Furthermore, a highly porous structure allows for increased formationcurrents and rates, which increases production throughput and lessensthe entrapment of undesirable electrolyte constituents. Moreover, theporous anode network improves conductivity within the final assembledwet capacitor. The resistance (ESR) within the capacitor is reduced bythe low anode density and open pores. This lower resistance results inhigher efficiencies.

Table 4 below demonstrates the difference in performance properties ofanodes from the prior art QR-3 tantalum powder in comparison to tantalumfibers according to the present invention.

TABLE 4 37 C 37 C DC Volume Charge AC Cap ESR Eout1 working Powder (cc)Wt (microF) (Ohms) (J) J/cc J/g voltage Fiber 0.866 5.5 120.86 6.84 9.3310.76 1.70 400 V Fiber 0.873 5.5 123.20 6.91 9.48 10.86 1.72 400 V QR-30.859 6.5 94.74 6.23 7.05 8.21 1.08 400 V Flake QR-3 0.854 6.5 94.686.57 7.06 8.27 1.09 400 V Flake

Referring now to FIGS. 15 and 16, an exemplary capacitor 100 accordingto the present invention is shown. The capacitor 100 comprises one ormore anodes of an anode active material and a cathode or cathodes of acathode active material facing adjoining anode surfaces and housedinside a hermetically sealed casing 102. The capacitor electrodes areoperatively associated with each other by a working electrolyte (notshown) contained inside the casing.

The casing 102 comprises first and second metal casing members 104 and106. The metal casing portions 104, 106 are preferably selected from thegroup consisting of tantalum, titanium, nickel, molybdenum, niobium,cobalt, stainless steel, tungsten, platinum, palladium, gold, silver,copper, chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron,and mixtures and alloys thereof. Preferably, the casing portions 104,106 have a thickness of about 0.001 to about 0.015 inches.

First casing member 104 is of a drawn metal structure comprising a firstface wall 108 joined to a surrounding side wall 110 extending to an edge112. Additionally, casing portion 104 can be of a machined constructionor be formed by a metal injection molding process. Second casing member106 is in the shape of a plate and comprises a second face wall 114having a surrounding edge 116. The casing members 104 and 106 arehermetically sealed together by welding the overlapping edges 112 and116 where they contact each other. The weld 118 is provided by anyconventional means; however, a preferred method is by laser welding.

A feedthrough 120 electrically insulates an anode terminal wire 122 fromthe casing 12. The terminal wire 122 extends from within the casing 102to the outside thereof. The location of a hole 124 in the surroundingside wall 110 of the casing member 104 into which the feedthrough 120 ismounted is either offset towards the front edge 112, offset towards theface wall 108, or centered between the face wall 108 and the front edge112. The hole 124 is in alignment with an embedded wire of one of theanodes, as will be described subsequently.

According to an exemplary embodiment, the feedthrough 120 comprises aninsulating material 128, for example glass, ceramic, polymer, or epoxy.Regardless the insulating material, the feedthrough 120 comprises aferrule 126 defining an internal cylindrical through bore or passage ofconstant inside diameter. The insulating material 128 seals between abore of the ferrule 126 and the anode terminal wire 122 passingtherethrough from inside the casing 12. The feedthrough 120 may alsoprevent material, for example, electrolyte from escaping the casing 12and prevent foreign material from entering the casing 12 in the locationof the anode terminal wire 122. The terminal wire 122 has a J-shapedinterior portion 130 for connection to one or more anode wires withincasing 102. A suitable glass for insulative seal 128 is, for example,ELAN® type 88 or MANSOL™ type 88.

Capacitor 100 further comprises an anode assembly one or more anodesmade as previously described and connected to the terminal wire 122 offeedthrough 120 within the casing 102. The anode assembly includes afirst anode pellet 132 and a second anode pellet 134. The first anodepellet 132 comprises an inner major face wall 136 and an outer majorface wall 138, both extending to a surrounding edge 140. Similarly, thesecond anode pellet 134 comprises an inner major face wall 142 and anouter major face wall 144, both extending to a surrounding edge 146.

An anode wire 148 that is partially embedded in the first anode pellet132 has a distal end 148A that is electrically connected to the J-shapedinterior portion 130 of the terminal wire 122. The anode wire 148 ispreferably of tantalum. As previously described, the tantalum anodepellets 132 and 134 are sintered under a vacuum at high temperatures andthen anodized in a suitable electrolyte. The anodizing electrolyte fillsthe pores of the tantalum pellets 132, 134 and a continuous dielectricoxide is formed thereon. In that manner, the anode pellets 132, 134 andextending wire 148 are provided with a dielectric oxide layer formed toa desired working or formation voltage.

A second U-shaped wire has an end portion 150A embedded in the firstpellet 132 and a second end portion 150B embedded in the second pellet134. The second wire has an exposed U-shaped portion 150C. Thus, theU-shaped anode wire is not directly connected to the terminal wire 122or to the wire 148 of anode pellet 132. Instead, it connects directly tothe first and second anode pellets 132, 134, and continuity to theembedded wire 148 is through the active material of the first anodepellet 132. In this manner, the anode pellets 132 and 134 are connectedto terminal wire 122 in series.

After the anode pellet 132 and extending wire 148 are anodized to thedesired formation voltage, the dielectric oxide is removed from thewire. The wire 148 is subsequently connected to an anode lead 122supported in an insulative material of the insulative seal 120. Laserwelding secures the wire 122 and lead 148 together. The wire 148 andconnected lead 122 are then re-anodized.

The U-shaped anode wire bridging between anode pellets 132, 134 and thefeedthrough of wire 122 including its J-shaped portion 130 joined to thedistal end 148A of wire 148 is enclosed and immobilized within a moldedpolymer (not shown). The various anode wires, whether embedded or not,are preferably positioned near the central regions of the respectiveanode pellets 132 and 134, i.e., equidistant from the inner and outerface walls of the pellets.

The cathode of capacitor 100 comprises cathode active material supportedby and in contact with the face walls of the casing members 102 and 104.More particularly, cathode active material contacts the inner surfacesof the respective casing face walls 108 and 114 in a pattern thatgenerally mirrors the shape of the anode pellets 132 and 134. Thecathode active material has a thickness of about a few hundred Angstromsto about 0.1 millimeters and is either directly coated on the inner,surfaces of the face walls 108, 114 or it is coated on a conductivesubstrate (not shown) supported on and in electrical contact with theinner surfaces thereof.

Another portion of the cathode active material is positionedintermediate the anodes 132 and 134. The intermediate cathode activematerial is supported on opposed surfaces of a cathode current collector152, preferably in the form of a foil. That way, the cathode currentcollector 152 having opposed first and second major faces provided withcathode active material thereon is positioned opposite the first andsecond anodes 132 and 134, thereby forming an anode-cathode assembly. Atab 152A is provided on current collector 152 for tack welding to theinner surface of the face wall 108, surrounding side wall 110 of casingmember 104, or to the second face wall 114. The tab 152A is bentapproximately perpendicular to the respective surrounding edges 140 and146 of anode pellets 132 and 134 to position it for welding to side wall110. The casing 102 comprising members 104, 106 serves as the cathodeterminal.

In that respect, the face walls 108, 114 of the casing portions 132, 134may be of an anodized-etched conductive material, have a sintered activematerial with or without oxide contacted thereto, be contacted with adouble layer capacitive material, for example a finely dividedcarbonaceous material such as graphite, activated carbon, carbon orplatinum black, a redox, pseudocapacitive or an under potentialmaterial, or be an electroactive conducting polymer such as polyaniline,polypyrrole, polythiophene, polyacetylene, and mixtures thereof.

According to one preferred aspect of the present invention, the redox orcathode active material includes an oxide of a first metal, the nitrideof the first metal, the carbon nitride of the first metal, and/or thecarbide of the first metal, the oxide, nitride, carbon nitride andcarbide having pseudocapacitive properties. The first metal ispreferably selected from the group consisting of ruthenium, cobalt,manganese, molybdenum, tungsten, tantalum, iron, niobium, iridium,titanium, zirconium, hafnium, rhodium, vanadium, osmium, palladium,platinum, nickel, and lead.

A pad printing process as described in U.S. Pat. No. 7,116,547 is onemethod for making such a coating. An ultrasonically generated aerosol asdescribed in U.S. Pat. Nos. 5,894,403, 5,920,455, 6,224,985, and6,468,605, all to Shah et al., is also a suitable deposition method.These are assigned to the assignee of the present invention andincorporated herein by reference. Other processes for depositing cathodematerial that are useful with the present invention include screenprinting as well as ink jet printing. Standard aerosol spraying can alsobe used.

The capacitor 100 preferably comprises separators of electricallyinsulative material that completely surround and envelop the anodepellets 132, 134. For example, a first separator 154 encloses the firstanode 132 and a second separator 156 encloses the second anode pellet134. The separators 154, 156 may be formed as pouches that enclose therespective anode pellets 132, 134. In particular, separator 154 issealed at a flap 158 of material that extends around the majority of theperimeter of anode pellet 132 except at the feedthrough wire 148A andembedded wire 150A. In like manner, separator pouch 156 is sealed at aflap 160 of material that extends around the majority of the perimeterof anode pellet 134 with anode wire 150B extending therefrom. Theindividual sheets of separator material are closed at flaps 158 and 160by a process such as ultrasonic welding, or heat sealing.

The separators 154 and 156 prevent an internal electrical short circuitbetween the anode and cathode active materials in the assembledcapacitor and have a degree of porosity sufficient to allow ion flowtherethrough during the charge and discharge of the capacitor 100.Illustrative separator materials include woven and non-woven fabrics ofpolyolefinic fibers including polypropylene and polyethylene orfluoropolymeric fibers including polyvinylidene fluoride,polytetrafluoroethylene, and polyethylenechlorotrifluoroethylenelaminated or superposed with a polyolefinic or fluoropolymericmicroporous film, non-woven glass, glass fiber materials and ceramicmaterials. Additional separator materials may include films of polysulfone and polyester, for example, polyethylene terephthalate. Suitablemicroporous films include a polyethylene membrane commercially availableunder the designation SOLUPOR® (DMS Solutech), a polytetrafluoroethylenemembrane commercially available under the designation ZITEX® (ChemplastInc.) or EXCELLEPATOR® (W. L. Gore and Associates), a polypropylenemembrane commercially available under the designation CELGARD® (CelanesePlastic Company, Inc.), and a membrane commercially available under thedesignation DEXIGLAS® (C. H. Dexter, Div., Dexter Corp.). Cellulosebased separators are also useful. Depending on the electrolyte used, theseparator 18 can be treated to improve its wettability, as is well knownby those skilled in the art. A preferred separator structure 18comprises a non-woven layer of polyethylene or polypropylene, amicroporous layer of polyethylene or polypropylene, and, possibly athird layer of polyethylene or polypropylene, which is also non-woven.Regardless its material of construction, the separator must be protectedfrom the heat generated when casing portion 104 is secured to casingportion 106 by weld 118.

In a final step of providing capacitor 100, the void volume in casing102 is filled with a working electrolyte (not shown) through a fillopening 162. This hole is then welded closed to complete the sealingprocess. A suitable working electrolyte for the capacitor 10 isdescribed in U.S. Pat. No. 6,219,222 to Shah et al., which includes amixed solvent of water and ethylene glycol having an ammonium saltdissolved therein. U.S. Pat. No. 6,687,117 to Liu and U.S. PatentApplication Pub. No. 2003/0090857 describe other electrolytes for thepresent capacitor 100. The electrolyte of the latter publicationcomprises water, a water-soluble inorganic and/or organic acid and/orsalt, and a water-soluble nitro-aromatic compound while the formerrelates to an electrolyte having de-ionized water, an organic solvent,isobutyric acid and a concentrated ammonium salt. These patents andpublication are assigned to the assignee of the present invention andincorporated herein by reference.

While capacitor 100 has been described as comprising cathode currentcollector 152 supporting cathode active material on its opposite majorsides and positioned intermediate the parallel connected anodes 132 and134 that is by way of example. Those skilled in the art will readilyunderstand that a capacitor according to the present invention canfurther have three or more to “n” anodes connected in parallel with eachother by bridging U-shaped anode wires (150A, 150B and 150C). There willbe cathode active material supported on the inner surfaces of the casingwalls 108, 114 and facing the first and the n^(th) anodes.

For a more detailed description of an exemplary capacitor useful withthe tantalum fibers according to the present invention, reference ismade to U.S. Pat. No. 7,483,260 to Ziarniak et al. The '260 patent isassigned to the assignee of the present invention and incorporatedherein by reference.

Thus, it should be apart to those of ordinary skill in the art that theuniqueness of the present invention is the ability to press an anodepellet having good integrity, good pore structure and that is capable ofbeing formed at high voltages. Resultant from this is the ability todesign and fabricate capacitors with higher voltages vs. the prior art.That is because the anodes of the present invention have higher energydensity and substantially low ESR in comparison to those made accordingto the prior art. This is achieved by the present invention through theuse of the starting tantalum fiber material and the specific processingthat creates this anodized anode body.

Although several embodiments of the invention have been described indetail, for purposes of illustration, various modifications of each maybe made without departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited, except as by theappended claims.

What is claimed is:
 1. A capacitorassembly, comprising: a) a casingcomprising first and second inner wall surfaces; b) at least a firsttantalum anode housed inside the casing, wherein: i) the first tantalumanode is characterized as having been fabricated from tantalum fibershaving a diameter ranging from 0.5 μm to 2.5 μm and a length rangingfrom 5 μm to 50 μm so that the tantalum fibers have a length: diameter(L/D) aspect ratio ranging from 2 to 100, the tantalum fibers forming atantalum powder as a loosely packed mass of the tantalum fibers, and ii)wherein the tantalum powder is characterized as having been agglomeratedinto a randomly oriented, substantially non-aligned, porous agglomeratedtantalum powder having a primary porosity density ranging from 1.5 g/ccto 4.5 g/cc, which agglomerated tantalum powder is characterized ashaving been pressed into a tantalum pellet having a secondary porositydensity ranging from 4 g/cc to 6.5 g/cc, and iii) wherein the tantalumpellet is characterized as having been sintered so that the sinteredtantalum pellet has: A) an inter-granule pore size distributionattributed to the primary porosity of a sintered density at: d10 ofranging from 0.5 μm to 2 μm; and d90 of ranging from 3 μm to 10 μm; andB) an intra-granule pore size distribution attributed to the secondaryporosity of sintered density at: d10 of ranging from 20 μm to 40 μm; andd90 of ranging from 60 μm to 100 μm; and iv) the sintered pellet ischaracterized as having then been anodized to a formation voltagegreater than zero up to 550 V to form a dielectric oxide on the tantalumfibers and thereby provide the tantalum anode; c) an insulative sealcomprising a feedthrough wire electrically connected to the firsttantalum anode, wherein the feedthrough wire extends outside the casingand is electrically isolated from the casing; d) a cathode comprisingcathode active material supported by and in electrical contact with thefirst inner wall surface and with the second inner wall surface of thecasing; e) a separator preventing direct physical contact between thefirst tantalum anode and the cathode active material supported on thefirst and second inner wall surfaces of the casing while allowing forion flow therethrough; and f) an electrolyte contacting the cathode andthe first anode.
 2. The capacitor of claim 1, including a secondtantalum anode housed inside the casing, wherein the second tantalumanode is electrically connected in parallel with the first tantalumanode, and wherein a cathode current collector comprising opposed majorcurrent collector faces supporting the cathode active material isdisposed intermediate the first and second tantalum anodes.
 3. Thecapacitor of claim 1, wherein there are “n” tantalum anodes electricallyconnected in parallel with each other and housed inside the casing, andwherein there are n−1 cathode current collectors supporting the cathodeactive material on opposed major current collector faces thereof anddisposed intermediate side-by-side adjacent ones of the n tantalumanodes.
 4. The capacitor of claim 1, wherein the casing comprises afirst casing member having a first face wall joined to a surroundingside wall and a second casing member having a second face wall securedto the surrounding side wall of the first casing member.
 5. Thecapacitor of claim 1, wherein the feedthrough wire is connected to thefirst tantalum anode and a wire extends from the first tantalum anode tothe second tantalum anode to electrically connect the first and secondtantalum anodes in parallel.
 6. The capacitor of claim 1 wherein thetantalum fibers comprising the tantalum powder have a length-to-diameter(L/D) aspect ratio ranging from 2 to
 100. 7. The capacitor of claim 1,wherein the tantalum fibers comprising the tantalum powder have a L/Daspect ratio ranging from 10 to
 40. 8. The capacitor of claim 1, whereinthe agglomerated tantalum powder has substantially no fines less than200 mesh.
 9. The capacitor of claim 1 wherein, prior to being sinteredand anodized to form the first tantalum anode, the tantalum pellet has apressed density of 3 g/cc to 8 g/cc.
 10. The capacitor of claim 1wherein, prior to being sintered and anodized to form the first tantalumanode, the tantalum pellet has a pressed density of 4 g/cc to 6.5 g/cc.11. The capacitor of claim 1, wherein the tantalum pellet ischaracterized as having been sintered at a temperature ranging from1,200° C. to 2,200° C., for a time ranging from 0.1 to 120 minutes, andat a vacuum of <1×10⁻⁴ Torr (Argon).
 12. The capacitor of claim 1,wherein the tantalum pellet is characterized as having been sintered ata temperature ranging from 1,500° C. to 1,850° C., for a time rangingfrom 1 to 10 minutes, and at a vacuum of <1×10⁻⁵ Torr (Argon).
 13. Thecapacitor of claim 1, wherein, prior to being anodized to form the firsttantalum anode, the sintered tantalum pellet has a the sintered densityranging from 4 g/cc to 9 g/cc.
 14. The capacitor of claim 1, wherein,prior to being anodized to form the first tantalum anode, the sinteredtantalum pellet has a the sintered density ranging from 5 g/cc to 8g/cc.
 15. The capacitor of claim 1, wherein, prior to being anodized toform the first tantalum anode, the sintered tantalum pellet has a d50 aninter-granule pore size distribution attributed to the primary porosityof sintered density at d50 ranging from 1 μm to 5 μm, and a d50 anintra-granule pore size distribution attributed to the secondaryporosity of sintered density at d50 ranging from 30 μm to 80 μm.
 16. Thecapacitor of claim 1 wherein, prior to being pressed into the tantalumpellet, the agglomerated tantalum powder has an agglomerate densityranging from 1.5 g/cc to 4.5 g/cc.
 17. The capacitor of claim 1,wherein, prior to being pressed into the tantalum pellet, theagglomerated tantalum powder is has an agglomerate a primary densityranging from 2.5 g/cc to 3.5 g/cc.
 18. The capacitor of claim 1,wherein, prior to being pressed into the tantalum pellet, theagglomerated tantalum powder has an agglomerate diameter of distributionat: i) d10 of 74 μm; ii) d50 ranging from 200 μm to 500 μm; and iii) d90of 1,000 μm.
 19. The capacitor of claim 1, wherein, prior to beingpressed into the tantalum pellet, the agglomerated tantalum powder has ad50 inter-granular pore size attributed to the primary porosity ofagglomerate pore size ranging from 1 μm to 5 μm.
 20. The capacitor ofclaim 1, wherein, prior to being pressed into the tantalum pellet, theagglomerated tantalum powder has a d50 inter-granular pore sizeattributed to the primary porosity of agglomerate pore size ranging from2 μm to 3 μm.
 21. The capacitor of claim 1, wherein, prior to beingpressed into the tantalum pellet, the agglomerated tantalum powder hasan inter-granular pore size attributed to the primary porosity ofagglomerate pore size distribution at d10 of 0.5 μm and at d90 of 20 μm.22. The capacitor of claim 1, wherein the sintered tantalum pellet ischaracterized as having been anodized in an electrolyte comprising H₃PO₄and having a conductivity up to about 20,000 μS at 40° C.
 23. Acapacitorassembly, comprising: a) a casing comprising first and secondinner wall surfaces; b) at least a first tantalum anode and a secondtantalum anode electrically connected in parallel and housed inside thecasing, wherein: i) the parallel connected first and second tantalumanodes are characterized as each having been fabricated from tantalumfibers having a diameter ranging from 0.5 μm to 2.5 μm and a lengthranging from 5 μm to 50 μm so that the tantalum fibers have a length:diameter aspect ratio ranging from 2 to 100, the tantalum fibers forminga tantalum powder as a loosely packed mass of the tantalum fibers, andii) wherein the tantalum powder is characterized as having beenagglomerated into a randomly oriented, substantially non-aligned, porousagglomerated tantalum powder having a primary porosity density rangingfrom 1.5 g/cc to 4.5 g/cc, which agglomerated tantalum powder ischaracterized as having been pressed into a first tantalum pellet andinto a second tantalum pellet, both pellets having a secondary porositydensity ranging from 4 g/cc to 6.5 g/cc, and iii) wherein the first andsecond tantalum pellets are characterized as having been sintered sothat the sintered first and second tantalum pellets each have: A) aninter-granule pore size distribution attributed to the primary porosityof a sintered density at: d10 of ranging from 0.5 μm to 2 μm; and d50ranging from 1 μm to 5 μm; and d90 of ranging from 3 μm to 10 μm; and B)an intra-granule pore size distribution attributed to the secondaryporosity of sintered density at: d10 of ranging from 20 μm to 40 μm; andd50 ranging from 30 μm to 80 μm; and d90 of ranging from 60 μm to 100μm; and iv) the sintered first and second tantalum pellets arecharacterized as having then been anodized to a formation voltagegreater than zero up to 550 V to form a dielectric oxide on the tantalumfibers and thereby provide the first and second tantalum anodes; c) aninsulative seal comprising a feedthrough wire electrically connected toat least the first tantalum anode, wherein the feedthrough wire extendsoutside the casing and is electrically isolated from the casing; d) acathode comprising cathode active material supported by and inelectrical contact with the first inner wall surface and with the secondinner wall surface of the casing; e) a cathode current collectorcomprising opposed major current collector faces supporting the cathodeactive material and being disposed intermediate the first and secondtantalum anodes; f) a separator material preventing direct physicalcontact between the first and second tantalum anodes and the cathodeactive material supported on the first and second inner wall surfaces ofthe casing, and between the first and second tantalum anodes and thecathode active material supported on the cathode current collector, theseparator material allowing for ion flow through; and g) an electrolytecontacting the cathode and the first and second anodes.
 24. Acapacitorassembly, comprising: a) a casing comprising first and secondinner wall surfaces; b) “n” tantalum anodes electrically connected inparallel and housed inside the casing, wherein: i) the parallelconnected n tantalum anodes are each characterized as having beenfabricated from tantalum fibers having a diameter ranging from 0.5 μm to2.5 μm and a length ranging from 5 μm to 50 μm so that the tantalumfibers have a length: diameter aspect ratio ranging from 2 to 100, thetantalum fibers forming a tantalum powder as a loosely packed mass ofthe tantalum fibers, ii) wherein the tantalum powder is characterized ashaving been agglomerated into a randomly oriented, substantiallynon-aligned, porous agglomerated tantalum powder having a primaryporosity density ranging from 1.5 g/cc to 4.5 g/cc, which agglomeratedtantalum powder is characterized as having been pressed into “n”tantalum pellets, each of the n pellets having a secondary porositydensity ranging from 4 g/cc to 6.5 g/cc, and iii) wherein the n tantalumpellets are characterized as having been sintered so that the sinteredtantalum pellets have: A) an inter-granule pore size distributionattributed to the primary porosity of a sintered density at: d10 ofranging from 0.5 μm to 2 μm; and d50 ranging from 1 μm to 5 μm; and d90of ranging from 3 μm to 10 μm; and B) an intra-granule pore sizedistribution attributed to the secondary porosity of sintered densityat: d10 of ranging from 20 μm to 40 μm; and d50 ranging from 10 μm to 80μm; and d90 of ranging from 60 μm to 100 μm; and iv) the sinteredpellets are characterized as having then been anodized to a formationvoltage greater than zero of at least 300 V up to 550 V to form adielectric oxide on the tantalum fibers and thereby provide the ntantalum anodes; c) an insulative seal comprising a feedthrough wireelectrically connected to at least a first one of the n tantalum anodes,wherein the feedthrough wire extends outside the casing and iselectrically isolated from the casing; d) a cathode comprising cathodeactive material supported by and in electrical contact with the firstinner wall surface and with the second inner wall surface of the casing;e) n−1 cathode current collectors housed inside the casing, each cathodecurrent collector comprising opposed major current collector facessupporting cathode active material and being disposed intermediateside-by-side adjacent ones of the n tantalum anodes; f) a separatormaterial preventing direct physical contact between the tantalum anodesand the cathode active material supported on the first and second innerwall surfaces of the casing, and between the tantalum anodes and thecathode active material supported on the n−1 cathode current collectors,the separator material allowing for ion flow through; and g) anelectrolyte contacting the cathodes and the anodes.
 25. A tantalum anodefor incorporation into a capacitor, the tantalum anode comprising: a)tantalum fibers having a diameter ranging from 0.5 μm to 2.5 μm and alength ranging from 5 μm to 50 μm so that the tantalum fibers have alength: diameter aspect ratio ranging from 2 to 100, wherein thetantalum fibers form a tantalum powder as a loosely packed mass of thetantalum fibers, and b) wherein the tantalum powder is characterized ashaving been agglomerated into a randomly oriented, substantiallynon-aligned, porous agglomerated tantalum powder having a primaryporosity density ranging from 1.5 g/cc to 4.5 g/cc, which agglomeratedtantalum powder is characterized as having been pressed into a tantalumpellet having a secondary porosity density ranging from 4 g/cc to 6.5g/cc, and c) wherein the tantalum pellet is characterized as having beensintered so that the sintered tantalum pellet has: i) an inter-granulepore size distribution attributed to the primary porosity of a sintereddensity at: d10 of ranging from 0.5 μm to 2 μm; and d50 ranging from 1μm to 5 μm; and d90 of ranging from 3 μm to 10 μm; and ii) anintra-granule pore size distribution attributed to the secondaryporosity of sintered density at: d10 of ranging from 20 μm to 40 μm; andd50 ranging from 30 μm to 80 μm; and d90 of 60 μm to 100 μm; and d) thesintered pellet is characterized as having then been anodized to aformation voltage greater than zero of at least 300 V up to 550 V toform a dielectric oxide on the tantalum fibers and thereby provide thetantalum anode.
 26. A method for providing a tantalum anode for anelectrolytic capacitor, the method comprising the steps of: a) providingtantalum fibers having a diameter ranging from 0.5 μm to 2.5 μm and alength ranging from 5 μm to 50 μm so that the tantalum fibers have alength: diameter aspect ratio ranging from 2 to 100; b) providing atantalum powder as a loosely packed mass of the tantalum fibers; c)agglomerating the tantalum powder into a randomly oriented,substantially non-aligned, porous agglomerated tantalum powder having aprimary porosity density ranging from 1.5 g/cc to 4.5 g/cc; d) pressingthe agglomerated tantalum powder into a tantalum pellet of a desiredshape, the tantalum pellet having a secondary porosity density rangingfrom 4 g/cc to 6.5 g/cc; e) sintering the tantalum pellet into acoalesced body of the tantalum fibers to thereby provide a sinteredtantalum pellet having: i) an inter-granule pore size distributionattributed to the primary porosity of a sintered density at: d10 ofranging from 0.5 μm to 2 μm; and d50 ranging from 1 μm to 5 μm; and d90of ranging from 3 μm to 10 μm; and ii) an intra-granule pore sizedistribution attributed to the secondary porosity of sintered densityat: d10 of ranging from 20 μm to 40 μm; and d50 ranging from 30 μm to 80μm; and d90 of ranging from 60 μm to 100 μm; and f) anodizing thesintered tantalum pellet to a formation voltage greater than zero up to550 V to form a dielectric oxide on the tantalum fibers and therebyprovide the tantalum anode.
 27. The method of claim 26, includingproviding the tantalum fibers comprising the tantalum powder having aL/D aspect ratio ranging from 10 to
 40. 28. The method of claim 26,including providing the agglomerated tantalum powder havingsubstantially no fines less than 200 mesh.
 29. The capacitor of claim 1,wherein the sintered tantalum pellet is characterized as having beenanodized to a formation voltage that is at least 300 V up to 550 V. 30.The capacitor of claim 23, wherein the first and second sinteredtantalum pellets are characterized as having been anodized to aformation voltage that is at least 300 V up to 550 V.
 31. The method ofclaim 26, including anodizing the sintered tantalum pellet to aformation voltage that is at least 300 V up to 550 V.